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The present invention is directed to the general field of forming gallium arsenide (GaAs) semiconductor devices. More particularly, it is directed to forming GaAs Metal-Semiconductor Field Effect Transistors (MESFETs).
FIG. 1 illustrates a simplified structure of a conventional GaAs MESFET 100. The MESFET 100 has a GaAs substrate 102, a source region 104, a drain region 106, an n-type channel 108, and a p-type background region 110 and. A source electrode 112 is formed above the source region 104, a drain electrode 114 is formed above the drain region 106 and a gate electrode 116 is formed between the source and drain electrodes on a surface of the GaAs substrate, and above the n-type channel 108. As seen in FIG. 1, the gate electrode 116 is formed in a depressed area 118 formed in the upper surface of the device. When a voltage is applied to the gate electrode 116, the width of the n-type channel changes, thereby affecting the flow of current between the source electrode 112 and the drain electrode 114.
In conventional ion implanted, or epitaxially grown, GaAs MESFET devices, such as that depicted in FIG. 1, the channel 108 is doped uniformly between the source 104 and drain 106 regions. The result is that the p-type background forms a p-n junction with the n-type channel doping underneath the channel. When the MESFET 100 is used as an amplifier, it normally operates with high electrical field intensity in the gate-drain region. In high RF power amplifiers, the electrical field in the gate-drain region may be high enough to initiate impact ionization, in which both excessive electrons and holes are generated. In such case, the holes become trapped in the p-n junction, thereby forming a virtual back-gating, which results in a pinch-off the n-channel 108. This phenomenon is termed a power transient in RF amplifiers, which is detrimental to normal operation.
The present invention uses selective ion implantation techniques to create a GaAs MESFET device with non-uniform doping profiles in the conduction channel. In the Source-Gate region of the MESFET, a conventional p-type implantation is used as the background, and one or more n-type implantations form the conduction channel. In the Gate-Drain region of the device, there is either no, or a reduced, background p-type implantation, and the n-type implantation dose is also reduced, resulting in lower doping concentration between the gate and the drain.
The present invention is also directed to a method for forming a GaAs MESFET having non-uniform doping profiles in the conduction channel. This is accomplished by forming a lightly-doped first conduction channel of a first type, forming a moderately doped second conduction channel of the first type along a first portion of the first conduction channel, forming a background region of a second type beneath the second conduction channel, forming source and drain regions at opposite ends of the first conduction channel, forming source and drain contacts over corresponding source and drain regions, and forming a gate contact between the source and drain contacts, the gate contact being positioned approximately over an end of the second conduction channel.